1. Field of the Invention
This invention relates to a volatile data store with a backup storage system which is operable during a main power source failure. More particularly the invention relates to a shadow memory system in which a shadow memory is continually updated to reflect changes in data stored by the main memory so that in the event of a power failure only a small portion of the main memory data need be stored using backup power.
2. Discussion of the Prior Art
There are many data processing application in which it is important or essential to preserve data stored in a volatile memory or data store during the course of a main utility power failure. One approach that is sometimes used in critical situations is to provide a short term battery backup and an auxiliary power generator. The battery maintains the integrity of stored data while an engine driving the auxiliary generator is started and brought up to speed. While successful, this arrangement is too costly to be practical for most applications.
Another commonly used data protection approach is to rely exclusively upon a battery backup in the event of a power failure. In one alternative arrangement the battery backup is used to preserve the integrity of stored data during a power failure. However, large data stores consume large amounts of power and practical limitations on battery backup energy capacity limit data preservation to a few hours. If a power failure lasts more than a few hours critical data will be lost.
Another known battery backup alternative is to use battery power to "dump" volatile memory data into a nonvolatile memory such as a disk drive upon the occurrence of a power failure. Because of the large amount of energy required to preserve the data for a large memory, practical battery sizes limit preservation of data to one, or at most two, occurrences. Assuming that the backup batteries are rechargeable, it will typically take at least 24 hours to recharge them. However, it is not uncommon for power failures to occur in a rapid sequence of multiple failures and restarts. If more power failures occur than can be accommodated by the data preservation system, data will be lost.